How to texture a huge number of meshes in one uv?
The easiest way to get the UVs of all meshes of a complex model into one UV map would be using the Texture Atlas addon. It's in Blender's Community Addon Catalog and can be activated in "User Preferences - Add-ons" (Search for "atlas").



Once activated, you can find it in "Properties Window - Render - Texture Atlas".

For an in-depth tutorial I recommend watching Gleb Alexandrow's great tutorial on how to use the add-on.

In case you already have separate UV maps, I'd use the settings I described in this answer.

The difference between the add-on and Paul Gonet's method is that the major part of the tedious "join - unwrap - separate" process will be done automatically.